1.2 The Standard Model

summary

To understand the natural world around us, we look to its most fundamental constituents.

The Standard Model of particle physics, which describes particle interactions and their associated forces, has been verified over many different energy and length scales.
In fact, there is no other such theory in any area of science that covers so many phenomena over so many scales.
What we consider elementary and “primary” constituents, however, may very well change in the future.

Quantum field theory is the language of particle physics.

The most intuitive way to describe particle interactions is to use specialized graphs developed by Richard Feynman.
These Feynman diagrams typically describe an interaction between the particles on the left as initial states, and the particles on the right as final states.
This interaction is mediated by an exchange of a particle.
In the example diagram from the lecture, and electron and a positron annihilate through the mediation of a photon. The photon then gives a quark-antiquark pair, and a gluon is being emitted.
The photon is the particle that mediates electromagnetism, and the gluon is the particle that mediates the strong nuclear force, two of the fundamental forces of nature.
Particle physics and condensed matter can be described in many ways, including expansive formulae and matrix expressions, but Feynman diagrams remain one of the most essential ways to describe interactions.

The Standard Model is a blueprint for the universe.

From sophisticated mathematical expressions, we can interpret such phenomena as light, particle interactions, and so on.
But if we look closer at the Lagrangian for the Standard Model—that is, the equation that summarizes the dynamics of a system—we notice that there are no mass terms.
This is the theory we have for particles before they acquire mass. In the theory’s “natural” state, particles do not have intrinsic mass—something that of course is in violent disagreement with our physical observations, where things have weight.
Now we know that the dynamics of the theory give particles mass through the Higgs mechanism.

Nearly every constituent of our natural world can be described in one picture.

In the grid shown in the lecture, we have the quarks in the top block, the leptons in the bottom block, and on the right column, the bosons, which are the force mediators.
These categorizations were made according to the symmetries that these particles interact with, and their quantum mechanical properties, including mass and spin.
Quantum mechanical spin is an intrinsic form of angular momentum. Particles with half-integer spins (1/2, 3/2) are known as fermions, whereas integer spin (0, 1, 2) particles are bosons. They obey different sets of rules and behaviors.
The fundamental forces of nature— electromagnetism, the strong force, and the weak force—are mediated by the photon, the gluon, and the W and Z bosons, respectively.

Particles acquire mass through the Higgs mechanism.

When we talk about some particle A “decaying” into particles B, C, and so on, it does not mean that particle A contains the other particles. It is a quantum mechanical process—a transmutation from one quantum mechanical state to another.
The full Standard Model was first written in 1967, before the Higgs mechanism, field, or particle were discovered.
When we include the Higgs mechanism to the aforementioned “blueprint,” we now see many mass terms. What changed between these two equations is something called spontaneous symmetry breaking, the mechanism that allows particles to acquire mass.
Again, from this blueprint, the fundamental forces we see around us emerge.